Dead pack


Dave/Forum contributors:

I'm afraid I'm one more HP 41 user with a dead battery pack. My 41CX always had the N-size tray, but I also have a 41C with a rechargeable pack, which is about to die.

Are trays available? I thinks it's simpler to use N-size cells, Otherwise, I'll have to repair the dead pack.

Any advice is welcome.


As an alternative, you may want to consider converting the rechargeable battery casing to accept N cells or N size rechargeables instead of trying to repair/rebuild. I have recently done this to solve this problem.


What did you do with the innards you removed?

I hope you didn't discard them...


I carefully removed the built-in electronics and keep them in a safe place. Hopefully, I can get hold of the correct sized rechargeable cells so that I can use them in the future.



Thanks for your advice. How do I get the pack disassembled?


The method I use is to cut along the side joins with a knife. Then score the rear join quite deeply.

At this point you'll have a pack that is disconnected on three sides, and connected at the back.

Insert a screwdriver in the side near the hinge formed by the still conected side and twist. Hopefully the rear join will break leaving the pack in 2 pieces, with relativly little damage and fitting together nicely.

At this stage, I have not even bothered to glue the packs back together. I just weap insulation tape around them.

CAUTION: Inserting a pack that is NOT held together is FAR easier than removing it. This is especially true if you have used oversized cells (1/3 AA)



Thanks. I found out that 1/3 N size batteries are difficult to get, and that 1/3 AAA batteries need the holder to be perforated. Is that true?


1/3 AAA are slightly smaller and so there will be a little extra room around them.

1/3 AA are larger and require lots of effort and modification to the back. but give you 110 mAHr.

The 1/3 AAA (50mAHr) have about the same capacity as the original nicads (1/2 N -- 55 mAHr).




Can you recommend a good site to buy them?


In Australia I would suggest Batteries Plus.

You need someone who will make up a pack for you. Take the original pack into them so they can see where the leads have to come out, and in what direction.



Thanks. Do they have a web page?


For what it's worth, yes.

But you're not in Australia, are you?


Another good source for batteries is EVS Supply in Richardson Texas ( They have been able to rebuild anything that I've ever brought them (including some real weird stuff) and have supplied me with some REALLY obscure batteries over the years. They just rebuilt a HP-65 pack with 1300 maH metal hydrides for 16 bucks... it will run a program non-stop for 12 hours now. If you can get 1/3 AAA metal hydride cells they will more than make up for the loss of capacity going from the larger sub-N cells.


NiMH are great, but how are you going to charge them?

The chargers in these calculators are not designed for them and will damage them in fairly short order


NiMH cells will work fine any just about any trickle charged NiCD application without any modifications whatsoever. Because they have about twice the capacity of NiCD batteries their charge rate (in cell capacity/charge current) will be about half of the NiCDs and take twice as long to charge fully. This means they are also stressed about half of what a NiCD would be. A typical cell can stand an continuous trickle charge rate of C/10 without problems (although I prefer less). In these calculator applications the NiMH cells have a charge rate of around C/25 or less. I have used NiMH cells in EVERY trickle charge replacement I've done in the last three years without a single failure. NiMH cells do not have the "memory effect" problems of NiCDs and are more enviromentally friendly. In the past I have built some solar powered remote sensing applications that have run continuously for over 10 years without losing a single bit of data or had any maintenance(except when Bubba with his rifle decides to do some target practice) and am rather familiar with long term reliable operation of rechargeable batteries.


An issue is that it seems that many HP calculators and accessories that I've seen seem to charge the nicads at about C/4


You can charge at C/4 IF AND ONLY IF your charging circuit detects the battery charge status and adapt to it (i.e.: entering a trickle charging mode when battery is almost full). If the charger is not "intelligent", then it is not advisable to charge at more than C/10.

Most current models of cell phones, laptops and PDAs do charge at more than C/10 in order to achieve recharging times of 2 hours or so, but "smart" chargers and batteries are required. Hint: see how many contacts a battery pack has, "smart" battery packs may even have a microprocessor on-board... so it is usual for them to have at least positive, negative and one or more extra contacts to sense the battery status or to communicate with such on-board battery controller.

Keep in mind that, at least in Woodstocks, the charging circuit is absolutely not intelligent, but since it is a half wave rectifier, the real current applied is about 45% of the DC value... This may explain the C/4 vs. C/10 issue in that case.

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